This invention relates to exhaust-gas cleaning devices for an engine installed on a motor vehicle, for example.
Conventionally, the amount of nitrogen oxides (NOx) absorbed by a NOx-absorbing material is detected for making a judgment on deterioration of the NOx-absorbing material which is provided in an exhaust passage. Basically, the amount of NOx absorbed by the NOx-absorbing material can be determined by examining the concentration of oxygen in exhaust gases that varies as NOx released from the NOx-absorbing material is reduced. However, since an exhaust-gas converting catalyst, in which the NOx-absorbing material is provided, occludes oxygen, it is impossible to say whether a change in the concentration of oxygen is caused by a release of the absorbed NOx or by a release of occluded oxygen by just detecting the change in oxygen concentration. It is therefore impossible to accurately determine the amount of the absorbed NOx. 
Under such circumstances, Japanese Unexamined Patent Publication No. 8-260949 proposes a deterioration detecting device for detecting deterioration of a NOx-absorbing material provided in an exhaust passage, in which the NOx-absorbing material absorbs NOx and stores oxygen when the air-fuel ratio of exhaust gas is high (lean exhaust gas), and releases the absorbed NOx and stored oxygen when the air-fuel ratio of the exhaust gas becomes low (rich exhaust gas). This deterioration detecting device comprises an air-fuel ratio sensor provided in the exhaust passage, downstream of the NOx-absorbing material, for generating an output signal corresponding to the air-fuel ratio of the exhaust gas, an air-fuel ratio switcher which switches the air-fuel ratio of the exhaust gas from a higher value to a lower value when the amount of NOx absorbed in the NOx-absorbing material is regarded as being almost zero, or more specifically, when the air-fuel ratio has been made high for a short time, and a stored oxygen quantity detector for detecting the amount of oxygen stored in the NOx-absorbing material based on the output signal generated by the air-fuel ratio sensor when the air-fuel ratio of the exhaust gas flowing into the NOx-absorbing material has been switched from a higher value to a lower value, wherein, after the air-fuel ratio has been made high for a long period by the air-fuel ratio switcher, the sum of the amount of oxygen stored in the NOx-absorbing material and the amount of NOx absorbed in the NOx-absorbing material is determined based on the output signal generated by the air-fuel ratio sensor when the air-fuel ratio of the exhaust gas flowing into the NOx-absorbing material has been switched from the higher value to the lower value, and the amount of NOx absorbed in the NOx-absorbing material is calculated by subtracting the amount of the stored oxygen from the aforementioned sum.
In the deterioration detecting device for detecting deterioration of the NOx-absorbing material disclosed in the aforementioned Publication, it is assumed that the period of time required for the NOx-absorbing material to store the greatest possible quantity (saturation level) of oxygen is sufficiently short and NOx is scarcely absorbed by the NOx-absorbing material during this time period. Based on this assumption, the deterioration detecting device of regards a value detected from a change in the air-fuel ratio of the exhaust gas that occurs when the air-fuel ratio is switched to a lower value after once being made high for a short time as being representative of the amount of the stored oxygen, and calculates the amount of NOx absorbed in the NOx-absorbing material from this detected value and the sum of the amount of the stored oxygen and the amount of NOx. Even if the aforementioned time period is short, however, the amount of NOx absorbed in the NOx-absorbing material is not zero and oxygen is produced when NOx released from the NOx-absorbing material is reduced. Thus, the deterioration detecting device of the Publication has a problem that its accuracy of judgment on deterioration of the NOx-absorbing material inevitably lowers due to errors occurring in detecting the amount of the stored oxygen.
An exhaust-gas cleaning device for an engine in which specific quantities of NOx-absorbing material and oxygen-absorbing material are provided in an exhaust passage also has a problem that it becomes impossible to accurately calculate the amount of absorbed NOx in a manner disclosed in the aforementioned Patent Publication when the functioning of the oxygen-absorbing material deteriorates.
In view of the foregoing, it is an object of the invention to provide an exhaust-gas cleaning device for an engine capable to making a judgment on deterioration of a NOx-absorbing material with a high degree of accuracy even when a specific quantity of NOx-absorbing material and a large quantity of oxygen-absorbing material are provided in an exhaust passage of the engine.
In a principal form of the invention, an exhaust-gas cleaning device for an engine comprises a NOx-absorbing material provided in an exhaust passage to absorb NOx under oxygen-rich conditions where the oxygen concentration is high and release absorbed NOx as the oxygen concentration decreases, an oxygen-absorbing material provided in the exhaust passage to absorb oxygen under oxygen-rich conditions where the oxygen concentration is high and release absorbed oxygen as the oxygen concentration decreases, an oxygen concentration controller for controlling the oxygen concentration in the exhaust passage, an oxygen concentration detector provided downstream of the NOx-absorbing material and the oxygen-absorbing material, an absorption quantity detector which determines a first amount of absorption of NOx and oxygen absorbed by the NOx-absorbing material and the oxygen-absorbing material based on a sensing signal fed from the oxygen concentration detector at a time when the oxygen concentration upstream of the NOx-absorbing material and the oxygen-absorbing material is just transferred from a high concentration state maintained for a preset first reference time to a low concentration state by controlling the oxygen concentration with the oxygen concentration controller, and determines a second amount of absorption of NOx and oxygen absorbed by the NOx-absorbing material and the oxygen-absorbing material based on a sensing signal fed from the oxygen concentration detector at a time when the oxygen concentration upstream of the NOx-absorbing material and the oxygen-absorbing material is just transferred from the high concentration state maintained for a preset second reference time which is longer than the first reference time to the low concentration state by controlling the oxygen concentration with the oxygen concentration controller, a NOx quantity estimator which obtains a first estimated value of the amount of NOx absorbed by the NOx-absorbing material during the first reference time and a second estimated value of the amount of NOx absorbed by the NOx-absorbing material during the second reference time in accordance with engine operating condition, and a deterioration detector which makes a judgment on deterioration of the NOx-absorbing material based on the first amount of absorption and the second amount of absorption of NOx and oxygen determined by the absorption quantity detector and the first estimated value and the second estimated value of the amount of absorbed NOx obtained by the NOx quantity estimator.
In the exhaust-gas cleaning device thus constructed, the judgment on deterioration of the NOx-absorbing material is made based on the first and the second amounts of absorption of NOx and oxygen determined by the absorption quantity detector and the first and the second estimated values of the amount of absorbed NOx obtained by the NOx quantity estimator. It is therefore possible to perform the deterioration judgment operation in a proper manner taking into account the amount of NOx absorbed in the NOx-absorbing material during the first reference time. When a three-way catalytic converter is provided upstream of the NOx-absorbing material, for instance, the three-way catalytic converter at a low temperature can be activated by reacting unburned fuel in exhaust gas with oxygen released from the oxygen-absorbing material. Modern exhaust-gas cleaning devices comprising a large quantity of oxygen-absorbing material in the three-way catalytic converter have a problem that the NOx-absorbing material tends to be wrongly judged undeteriorated despite the fact that it is already deteriorated, because a reference calculation time set for calculating the amount of oxygen absorbed in the oxygen-absorbing material required for saturating it can not be sufficiently shortened and a specific amount of NOx is absorbed by the NOx-absorbing material during the reference calculation time, making it impossible to accurately determine the amount of oxygen absorbed in the oxygen-absorbing material. It is possible to overcome this problem of the prior art by employing the aforementioned construction of the invention.
According to a unique feature of the invention, the exhaust-gas cleaning device is constructed such that the deterioration detector makes a judgment on deterioration of the NOx-absorbing material based on the difference between the second amount of absorption and the first amount of absorption of NOx and oxygen determined by the absorption quantity detector and the difference between the second estimated value and the first estimated value of the amount of NOx obtained by the NOx quantity estimator.
In this construction, the deterioration detector makes a judgment on whether or not a specified quantity of NOx is absorbed in the NOx-absorbing material in a proper manner based on, for example, the ratio of the difference between the second amount of absorption and the first amount of absorption of NOx and oxygen determined by the absorption quantity detector to the difference between the second estimated value and the first estimated value of the amount of NOx obtained by the NOx quantity estimator.
According to another feature of the invention, the NOx quantity estimator obtains the first estimated value and the second estimated value of the amount of NOx absorbed by the NOx-absorbing material in accordance with engine speed and engine load.
In this construction, the first estimated value and the second estimated value of the amount of NOx corresponding to the operating condition of the engine can be read from a map defined using the engine speed and engine load as parameters, for instance.
In another principal form of the invention, an exhaust-gas cleaning device for an engine comprises a NOx-absorbing material provided in an exhaust passage to absorb NOx under oxygen-rich conditions where the oxygen concentration is high and release absorbed NOx as the oxygen concentration decreases, an oxygen-absorbing material provided in the exhaust passage to absorb oxygen under oxygen-rich conditions where the oxygen concentration is high and release absorbed oxygen as the oxygen concentration decreases, an oxygen concentration controller for controlling the oxygen concentration in exhaust gas, an oxygen concentration detector provided downstream of the NOx-absorbing material and the oxygen-absorbing material, an absorption quantity detector which determines a first amount of absorption of NOx and oxygen absorbed by the NOx-absorbing material and the oxygen-absorbing material based on a sensing signal fed from the oxygen concentration detector at a time when the oxygen concentration upstream of the NOx-absorbing material and the oxygen-absorbing material is just transferred from a high concentration state maintained for a preset first reference time to a low concentration state by controlling the oxygen concentration with the oxygen concentration controller, and determines a second amount of absorption of NOx and oxygen absorbed by the NOx-absorbing material and the oxygen-absorbing material based on a sensing signal fed from the oxygen concentration detector at a time when the oxygen concentration upstream of the NOx-absorbing material and the oxygen-absorbing material is just transferred from the high concentration state maintained for a preset second reference time which is longer than the first reference time to the low concentration state by controlling the oxygen concentration with the oxygen concentration controller, a first deterioration detector which makes a judgment on deterioration of the NOx-absorbing material based on the first amount of absorption and the second amount of absorption of NOx and oxygen determined by the absorption quantity detector, and a second deterioration detector which makes a judgment on deterioration of the oxygen-absorbing material, wherein the judgment on deterioration of the NOx-absorbing material by the first deterioration detector is inhibited when the oxygen-absorbing material has been judged to have deteriorated by the second deterioration detector.
In the exhaust-gas cleaning device thus constructed, the second deterioration detector judges whether or not the oxygen-absorbing material has deteriorated, and if it is judged undeteriorated, the first deterioration detector properly judges whether or not the NOx-absorbing material has deteriorated based on the second amount of absorption of NOx and oxygen absorbed by the NOx-absorbing material and the oxygen-absorbing material determined by the absorption quantity detector. On the other hand, when the oxygen-absorbing material is judged to have deteriorated, the judgment on deterioration of the NOx-absorbing material by the first deterioration detector is inhibited to thereby prevent misjudgment by the first deterioration detector potentially caused by the deterioration of the oxygen-absorbing material. Therefore, this construction serves to improve the accuracy of judgment on deterioration of the NOx-absorbing material by the first deterioration detector.
According to a feature of the invention, the absorption quantity detector is so constructed as to determine the amount of NOx and oxygen absorbed by the NOx-absorbing material and the oxygen-absorbing material based on the time period required for the oxygen concentration detected by the oxygen concentration detector to decrease to a specific low concentration level from a point in time when the oxygen concentration controller executes a control operation for transferring the oxygen concentration upstream of the NOx-absorbing material and the oxygen-absorbing material from the high concentration state to the low concentration state.
In this construction, the first and second amounts of absorption of NOx and oxygen absorbed by the NOx-absorbing material and the oxygen-absorbing material are obtained as large values when the time period required for the oxygen concentration detected by the oxygen concentration detector to decrease to the specific low concentration level from the point in time when the oxygen concentration controller executes the control operation for transferring the oxygen concentration upstream of the NOx-absorbing material and the oxygen-absorbing material from the high concentration state to the low concentration state is long. Contrary to this, when the aforementioned time period is short, the first and second amounts of absorption of NOx and oxygen absorbed by the NOx-absorbing material and the oxygen-absorbing material are obtained as small values.
According to another feature of the invention, the oxygen concentration controller is so constructed as to control the oxygen concentration in the exhaust passage by controlling the air-fuel ratio in a combustion chamber.
In this construction, the control operation for transferring the oxygen concentration of the exhaust gas discharged from the engine into the exhaust passage from the high concentration state maintained for the preset first reference time to the low concentration state and from the high concentration state maintained for the preset second reference time to the low concentration state is performed by controlling the air-fuel ratio in the combustion chamber with the oxygen concentration controller and, then, the absorption quantity detector determines the first and second amounts of absorption of NOx and oxygen.
According to still another feature of the invention, the oxygen-absorbing material is provided upstream of the NOx-absorbing material.
This construction makes it possible to improve exhaust-gas cleaning performance at low temperatures by using a large quantity of the oxygen-absorbing material. When the exhaust gas in the exhaust passage transfers from an oxygen-rich state to a state in which the oxygen concentration has decreased, oxygen released from the oxygen-absorbing material is supplied to the NOx-absorbing material downstream of the oxygen-absorbing material. Since oxygen produced when NOx released from the NOx-absorbing material is reduced and oxygen released from the oxygen-absorbing material together affect oxygen concentration values detected by the oxygen concentration detector, significant effects are obtained by making a judgment on deterioration of the NOx-absorbing material taking into account the amount of NOx absorbed by the NOx-absorbing material during the first reference time as stated above.
It will be recognized from the foregoing that the present invention confers such advantageous effects that an exhaust-gas cleaning device for an engine in which a specific quantity of NOx-absorbing material and a large quantity of oxygen-absorbing material are provided in an exhaust passage can make a judgment on deterioration of the NOx-absorbing material with a high degree of accuracy.
These and other objects, features and advantages of the invention will become more apparent upon a reading of the following detailed description in conjunction with the accompanying drawings.